Various types of touch probe coordinate measuring systems are known. In the type of touch probe coordinate measuring system under consideration here, the workpiece is measured by using a multi-camera vision system to determine the location of the touch probe when the touch probe tip is at a desired location on a workpiece surface. A visual marker pattern is located on the body of the touch probe, with the markers being imaged by at least two cameras of the vision system, and the images are used to triangulate the position of each of the markers in three dimensional space. Based on this data the probe tip location coordinates and the adjacent workpiece surface coordinates may be inferred or estimated.
Factors that limit the measurement accuracy of the type of touch probe measurement systems outlined above include errors that are introduced by distortions and/or erroneous assumptions regarding the coordinate frame associated with the multi-camera vision system. Such errors are referred to as camera frame distortion errors herein. Errors are also introduced by distortions and/or erroneous assumptions regarding the relationship between the marker locations in the probe tip location. Such errors are referred to as probe form errors herein.
U.S. Pat. Nos. 5,828,770, 5,805,287, and 6,497,134 each disclose various features related to the type of touch probe coordinate measuring system outlined above, and each is hereby incorporated by reference in its entirety. The '770 patent describes systems and methods related to performing measurements using an object (e.g. a probe) that includes a plurality of activatable markers. However, the '770 patent is generally not directed toward systems and methods for reducing camera frame distortion errors or probe form errors, and includes few, if any, teachings in this regard.
In contrast the '287 patent discloses a method for calibrating and/or correcting certain types of camera frame distortion errors. To briefly summarize that calibration method, the '287 patent teaches that: (i) the positions of permanently mounted light sources or reflectors are registered by their image on each camera, and their positions in the image are given as coordinates related to a camera fixed coordinate system; and (ii) the positions of at least two points for which the mutual separation distances are known are registered by holding a probing tool in contact with the points, and the positions of the points are calculated from the observed images of the light sources or reflectors of the probing tool. Based on the obtained data, the correct length scale in the camera frame may be established, and optical properties of the cameras may be mathematically modeled such that image distortions occurring through the camera lens may be compensated, all of which falls within the scope of calibrating and/or compensating camera frame distortion errors. However, the teachings of the '287 patent with regard to camera frame distortion errors do not encompass potential probe form errors, or their potential deleterious influence on the camera frame distortion calibration methods of the '287 patent.
The '134 patent discloses a method for calibrating and/or correcting a probe form error. In particular, the '134 patent addresses determining a location error for a feature of a surgical probe or instrument (e.g. its tip), relative to a set of energy emitters (e.g. markers) on its body. To briefly summarize the calibration method, the '134 patent teaches that the location error is found by: (i) calculating the position and orientation of the body having the energy emitters disposed thereon, in a plurality of orientations and positions relative to a reference frame, but with the feature (e.g. the tip) in a substantially constant position relative to the reference frame, (ii) calculating the locations of the feature of the object (e.g. the tip) from these calculated positions and orientations, (iii) averaging these calculated locations, (iv) determining the location of the feature by physical measurement thereof in relation to the physical locations of the emitters, and (v) comparing the calculated average location with the physically measured location to arrive at the error. In order to reduce or avoid the effects of camera frame distortion errors when determining probe form error, the teachings of the '134 patent include imaging a local reference frame that comprises an additional plurality of “fixed emitters”, at the same time that the “body emitters” are imaged. Calculating the positions and orientations of the “body emitters” relative to the additional “fixed emitters”, rather than relative to the overall camera frame, largely circumvents the effects of camera frame distortion errors. Otherwise, the teachings of the '134 patent with regard to calibrating or correcting probe form error do not encompass potential camera frame distortion errors, or their potential deleterious influence on probe form error calibration methods in the absence of additional “fixed emitters” in the calibration images.
As outlined above, a calibration method that efficiently encompasses camera frame distortion errors, as well as probe form errors, is not in evidence. Rather, separate calibration of these errors, using fixed reference emitters, or the like, is the norm. The present invention is directed to providing a system and method that overcomes the foregoing and other disadvantages.